The present invention relates to a servo control apparatus which may be advantageously incorporated into a facsimile transceiver, impact printer, magnetic disc head drive or the like.
U.S. Pat. No. 3,839,655 discloses a servo control system in which a magnetic transducer or sensor produces sinusoidal position signals in response to rotation of a servo motor shaft. These position signals are inverted, differentiated and commutated to produce an actual velocity signal which is compared with a velocity command signal to produce a motor drive signal.
A reference signal is produced by commutating the position signals and has a magnitude which is a function of the amplitude of the position signals. The reference signal is voltage divided in accordance with the distance of the motor shaft from the command position to produce the velocity command signal.
The reference signal and thereby the velocity command signal are produced in the form of ripple signals having cusps corresponding to the positive peaks of the position signals. It has been found in practice that erratic operation results due to the ripple component of the velocity command signal. It has therefore been proposed to generate the reference signal as a D.C. signal.
However, the output of the sensor array and thereby the position signals vary in amplitude due to high frequency attenuation, wow, flutter, ambient temperature variations and like factors. Thus, producing the reference signal as a constant D.C. signal introduces the problem of inaccurate speed control since the magnitude of the differentiated position signals is proportional to the motor shaft speed, and any errors in the amplitude of the position signals produce corresponding errors in the magnitude of the velocity signal.